<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
         function a(a) {
         for (i = 2;i <a; i++) {
             if (a % i === 0 ) {
                  return false
             } else {
                 return true
             }
         }
        }
        var b = a(5)
        if (b === true ) {
            alert('是素数')
        }else {
            alert('不是素数')
        }

    </script>
</body>
</html>